1. Field of the Invention
The present invention relates to digital communication systems. More particularly, the present invention relates to the enhancement of speech quality when portions of an encoded bit stream representing a speech signal are lost within the context of a digital communications system.
2. Background
In speech coding (sometimes called “voice compression”), a coder encodes an input speech signal into a digital bit stream for transmission. A decoder decodes the bit stream into an output speech signal. The combination of the coder and the decoder is called a codec. The transmitted bit stream is usually partitioned into segments called frames, and in packet transmission networks, each transmitted packet may contain one or more frames of a compressed bit stream. In wireless or packet networks, sometimes the transmitted frames or packets are erased or lost. This condition is typically called frame erasure in wireless networks and packet loss in packet networks. When this condition occurs, to avoid substantial degradation in output speech quality, the decoder needs to perform frame erasure concealment (FEC) or packet loss concealment (PLC) to try to conceal the quality-degrading effects of the lost frames. Because the terms FEC and PLC generally refer to the same kind of technique, they can be used interchangeably. Thus, for the sake of convenience, the term “packet loss concealment,” or PLC, is used herein to refer to both.
Today, a growing and popular wireless communications protocol being deployed is Bluetooth®, an industrial specification for wireless personal area networks (PANs). Bluetooth® provides a way to connect and exchange information between devices such as mobile phones, laptops, personal computers, printers, headsets, etc. over a secure, globally unlicensed short-range radio frequency.
On the Bluetooth® air-interface, a 64 kilobit/second (kb/s) log pulse code modulation (PCM) format (A-law or u-law) or a 64 kb/s Continuously Variable Slope Delta (CVSD) modulation format may be used for narrowband (8 kilohertz (kHz) sampling rate) speech signals. For higher sampling rates (e.g., 16, 32, or 44 kHz), the Low-Complexity Sub-band Codec (LC-SBC) may be used. LC-SBC is an audio coding system specially designed for Bluetooth® audio applications to obtain high quality audio at medium bit rates, and having a low computational complexity. As cellular telephone communication evolves to wideband speech, Bluetooth® headsets must also support wideband speech. LC-SBC is currently a mandatory codec in supporting wideband speech, but there is no PLC specification for LC-SBC.